It's not your toddler's fault that he had an accident. The potty is safe only when you're there. Otherwise, the potty monster visits. If he goes there instead of in his pants, it will snatch him and pull him in.
This isn't a real horror movie . . . yet. But it could be one. Andy Herald of How to Be a Dad has 6 great pitches to offer producers. They're terrify any toddler at bedtime, which is, of course, precisely what every parent of a toddler wants to do.